Many medical conditions, such as bone fracture, involve damages to the hard tissues (e.g., bones). Such conditions need materials that can be used to repair the hard tissue damages. With increasing life expectancy, the need for such materials is expected to increase substantially.
The materials used in such repairs often need to have sufficient mechanical strength to substitute for the functions of the damage hard tissues. These materials may be used temporarily, i.e., until the hard tissue repairs itself, or they may be used as permanent replacements. Various materials used in such hard tissue repairs include ceramic materials.
Currently, there are three types of ceramics that are clinically used for such purposes. Bioactive ceramics are materials that can directly bond with host bone. Examples of such materials include hydroxyapatite (Ca10(PO4)6(OH)2; HAp). The second type of bioceramics is biodegradable ceramics. These materials can be gradually resorbed in the body. Such biodegradable materials include, for example, tri-calcium phosphates (Ca3(PO4)2; TCP). The third type of bioceramics is bio-inert ceramics. These materials are stable in the living body and have high mechanical strength. Examples of bio-inert ceramic materials include alumina (α-Al2O3) and tetragonal zirconium (t-ZrO2).
Hydroxylapatite is a natural composition found in teeth and bones within the human body. Thus, hydroxyapatite (HAp) has an excellent biocompatibility and, therefore, would be a good candidate material for hard tissue replacements or repairs. Indeed, it is commonly used as a filler to replace damaged bone or as a coating to promote bone in-growth on prosthetic implants. Some medical implants, e.g. hip replacements or dental implants, are coated with hydroxyapatite, and it has been found that hydroxyapatite may promote osseo-integration of these artificial implants.
Because of these favorable properties of hydroxyapatite, there has been an immense interest in further developing and improving this material for medical use. Various methods have been disclosed for modifying hydroxyapatite and other implant materials to improve their bone adhesion and other properties. For example, it has been shown that coating this material with bone morphogenetic proteins can improve cell adhesion and subsequent tissue attachment. See, Zeng, H., et al., Biomaterials 20 (1999): 377 384. Another commonly used modification is nitridation, which improves the hardness of hydroxyapatite and its chemical inertia to the biological environment. See, Habelitz, S., et al., J. European Ceramic Society 19 (1999): 2685 2694, and Torrisi, L., Metallurgical Science and Technology 17(1) (1999): 27 32.
More recently, U.S. Pat. No. 7,211,271 issued to Risbud et al., combines these two approaches (i.e., nitridation and coating with a bone morphogenetic protein or an analog thereof, or DNA encoding such a protein or analog) to produce hydroxyapatite that facilitates the growth of tissues on such materials.
Although hydroxyapatite and modified hydroxyapatite materials have excellent biocompatibilities and beneficial tissue/bone formation stimulating effects, the mechanical strengths, especially the toughness value and Young's modulus, of hydroxyapatite materials are substantially different from those of living cortical bones. As a result, the use of hydroxyapatite in bone repair or replacement may lead to undesired stress around the junctions of these artificial materials and the natural bones. Such undesired stress will eventually lead to junction failures. Thus, there remains a need for new materials that would have the benefits of hydroxyapatite, but with mechanical properties more similar to those of natural bones.